The invention is based on a method as generally defined hereinafter. Waste heat recovery by endothermic fuel gasification, e.g., methanol gasification, is known for instance from the SAE Technical Paper, Series 790427, entitled "Methanol Decomposition Bottoming Cycle for IC Engines". The methanol gasification proposed there provides for the gas from decomposition to be admixed with the aspirated air or charge air. However, although this provision makes it possible to improve the overall efficiency by a maximum of 20% as compared with operation with liquid fuel, nevertheless the maximum power or engine load is markedly less than the possible full load of a conventional Diesel or Otto engine, because the air supply rate is decreased to correspond with the quantity of gas supplied, so knocking may sometimes occur.